SPOC: synchronous and asynchronous modes of contraction. (A) SPOCs are depicted as a sawtooth pattern of rapid sarcomere lengthening followed by slow sarcomere shortening. Figure redrawn from Wolfe et al. (2011) with permission from Biophysical Reviews. (B) Synchronous twitch contraction typically found in vertebrate striated muscles such as heart is depicted as an increase and decrease in force (F) coupled to a corresponding increase and decrease in [Ca²⁺]. (C) Asynchronous twitch contractions representative of IFMs are shown as multiple cycles of contraction and relaxation in the presence of Ca²⁺. Note that during cycles of asynchronous contractions, relaxation is uncoupled from a decrease in [Ca²⁺]. Figure created with BioRender.com.